rbtree will crash node with out of memory if the comparison value is bigint

[thanpolas]

Title basically, I am using native JS bigint and it crashes the node process:

const { RBTree } = require('bintrees');

const tree = new RBTree((a, b) => a - b);

tree.insert(BigInt(1));
console.log('length one:', tree.size);

// the next insert will crash node
tree.insert(BigInt(2));
console.log('length two:', tree.size);